Nitrogen and Phosphorous Retention in Tropical Eutrophic Reservoirs with Water Level Fluctuations: A Case Study Using Mass Balances on a Long-Term Series

Nitrogen and phosphorous loading drives eutrophication of aquatic systems. Lakes and reservoirs are often effective N and P sinks, but the variability of their biogeochemical dynamics is still poorly documented, particularly in tropical systems. To contribute to the extending of information on tropical reservoirs and to increase the insight on the factors affecting N and P cycling in aquatic ecosystems, we here report on a long-term N and P mass balance (2003–2018) in Valle de Bravo, Mexico, which showed that this tropical eutrophic reservoir lake acts as a net sink of N (−41.7 g N m y) and P (−2.7 g P m y), mainly occurring through net sedimentation, equivalent to 181% and 68% of their respective loading (23.0 g N m y and 4.2 g P m y). The N mass balance also showed that the Valle de Bravo reservoir has a high net N atmospheric influx (31.6 g N m y), which was 1.3 times the external load and likely dominated by N fixation. P flux was driven mainly by external load, while in the case of N, net fixation also contributed. During a period of high water level fluctuations, the net N atmospheric flux decreased by 50% compared to high level years. Our results outlining water regulation can be used as a useful management tool of water bodies, by decreasing anoxic conditions and net atmospheric fluxes, either through decreasing nitrogen fixation and/or promoting denitrification and other microbial processes that alleviate the N load. These findings also sustain the usefulness of long-term mass balances to assess biogeochemical dynamics and its variability.This research was funded by UNAM, PAPIIT-IN207702 and CONACYT-SEMARNAT, C01-1125 projects to M.M-

Vertical Boundary Mixing Events during Stratification Govern Heat and Nutrient Dynamics in a Windy Tropical Reservoir Lake with Important Water-Level Fluctuations: A Long-Term (2001–2021) Study

hysical processes play important roles in controlling eutrophication and oligotrophication. In stratified lakes, internal waves can cause vertical transport of heat and nutrients without breaking the stratification, through boundary mixing events. Such is the case in tropical Valle de Bravo (VB) reservoir lake, where strong diurnal winds drive internal waves, boundary mixing, and hypolimnetic warming during stratification periods. We monitored VB during 21 years (2001–2021) when important water-level fluctuations occurred, affecting mixing and nutrient flux. Stability also varied as a function of water level. Hypolimnetic warming (0.009–0.028 °C day−1) occurred in all the stratifications monitored. We analyzed temperature distributions and modeled the hypolimnion heat budget to assess vertical mixing between layers (0.639–3.515 × 10−6 m3 day−1), vertical diffusivity coefficient KZ (2.5 × 10−6–13.6 × 10−6 m2 s−1), and vertical nutrient transport to the epilimnion. Nutrient flux from the metalimnion to the epilimnion ranged 0.42–5.99 mg P m−2day−1 for soluble reactive phosphorus (SRP) and 5.8–101.7 mg N m−2day−1 for dissolved inorganic nitrogen (DIN). Vertical mixing and the associated nutrient fluxes increase evidently as the water level decreases 8 m below capacity, and they can increase up to fivefold if the water level drops over 12 m. The observed changes related to water level affect nutrient recycling, ecosystemic metabolic balance, and planktonic composition of VB.This research was funded by UNAM (PAPIIT-IN207702 and PAPIIT-IN111321) and by CONACYT-SEMARNAT (C01-1125) projects to M.M.-I.Peer reviewe

Mexican Euplotes euryhalinus Cyanobacteria-Grazing Vs. Possible Symbiont Acquisition

A comprehensive analysis was performed with the 2003-2019 data from an athalassohaline maar-crater lake in Alchichica, Mexico (18°10'N; 93°10' W, 2340 m a.s.l.). Ciliate numbers were obtained using Quantitative Protargol Stain; the molecular identification of Euplotes euryhalinus (Valbonesi & Luporini 1990) was proven. Ingested ciliate food was analysed in DAPI stained preparations, while feeding rates were based on an in situ ingestion of Fluorescently Labelled Bacteria (Sherrs´ FLB method), prepared from Synechococcus sp. During 2018, dilution experiments were performed with 20 μm screen-harvested ciliates and 0.2 μm filtered water with low dissolved oxygen, DO and enriched carbon dioxide content (“candle-jar” treatment). E. euryhalinus used to be found throughout the water thermal stratification-period with the anoxic hypolimnion. Surprisingly, most of the ciliate were located either around the oxycline and/or in the well illuminated layers including the surface itself. During the lake mixing, the ciliate was not recorded. E. euryhalinus was observed either with ingested minute diatoms (Cyclotella choctawhatcheeana), green algae (Oocystis parva, Monoraphidium minutum) or other chloroplast-bearing eukaryotes in different levels of pigment degradation that nullify the previous hypothesis of ciliate mixotrophy in the lake. Statistical tests are being conducted to explore the possibility of a direct relation to the nanophytoplankton peaks as a food source. However, the ciliates were not almost observed in the layers with abundant photosynthetic anoxygenic bacteria. We had to re-interpret previously measured feeding rates upon picocyanobacteria, Pcy, although the values were confirmed in recent experiments. Furthermore, published “picocyanobacteria in feeding vacuoles” were observed in vivo being ingested as microcolonies. This explained why observed filtration rates were not related directly to the Pcy numbers and why the Pcy uptakes were far beyond the ciliate growth needs. In both environmental samples and in the dilution experiments, Pcy / FLB were not included in observable vacuoles, which could be related to the known acquisition of endosymbiotic bacteria by Euplotes spp. It was confirmed that Pcy observed inside the ciliate cells were obtained through two different mechanisms: (i) ingestion of colonies placed in vacuoles and, as a hypothesis, (ii) filtration feeding / acquisition of potential symbionts towards the cytoplasm